1. Field of the Invention
The invention relates to methods and apparatus for controlling the operation of xe2x80x9clean-burnxe2x80x9d internal combustion engines used in motor vehicles to obtain improved engine and/or vehicle performance, such as improved vehicle fuel economy or reduced overall vehicle emissions.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx ) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (xcex), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as HC, CO and NOx, based, for example, on detected values for instantaneous engine speed and engine load.
To limit the amount of feedgas constituents that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx xe2x80x9ctrapxe2x80x9d. Both the upstream and downstream catalyst store NOX when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and release previously stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry.
Significantly, each purge event is characterized by a fuel xe2x80x9cpenaltyxe2x80x9d consisting generally of an amount of fuel required to release both the oxygen stored in the three-way catalyst, and the oxygen and NOx stored in the trap. Moreover, the trap""s NOx-storage capacity is known to decline in a generally-reversible manner over time due to sulfur poisoning or xe2x80x9csulfurization,xe2x80x9d and in a generally-irreversible manner over time due, for example, to component xe2x80x9cagingxe2x80x9d from thermal effects and xe2x80x9cdeep-diffusionxe2x80x9d/xe2x80x9cpermanentxe2x80x9d sulfurization. As the trap""s capacity drops, the trap is xe2x80x9cfilledxe2x80x9d more quickly, and trap purge events are scheduled with ever-increasing frequency. This, in turn, increases the overall fuel penalty associated with lean engine operation, thereby further reducing the overall fuel economy benefit of xe2x80x9crunning lean.xe2x80x9d
In order to restore trap capacity, a trap desulfurization event is ultimately scheduled, during which additional fuel is used to heat the trap to a relatively-elevated temperature, whereupon a slightly-rich air-fuel mixture is provided for a relatively-extended period of time to release much of the stored sulfur and rejuvenate the trap. As with each purge event, each desulfurization event typically includes the further xe2x80x9cfuel penaltyxe2x80x9d associated with the initial release of oxygen previously stored in the three-way catalyst and the trap. Accordingly, the prior art teaches scheduling a desulfurization event only when the trap""s NOx-storage capacity falls below a critical level, thereby minimizing the frequency at which such further fuel economy xe2x80x9cpenaltiesxe2x80x9d are incurred.
Unfortunately, as a further impact of trap sulfurization, empirical data suggests that a trap""s instantaneous NOx-storage efficiency, i.e., its instantaneous ability to incrementally store NOx, is increasingly affected by trap sulfurization as the trap begins to fill with NOx. Specifically, while a trap""s instantaneous efficiency immediately after a trap purge event is believed to remain generally unaffected by trap sulfurization, the instantaneous efficiency begins to fall more quickly, and earlier in the fill event, with increasing trap sulfurization. Such reduced trap efficiency leads to increased instantaneous NOx emissions, even when the trap is not yet xe2x80x9cfilledxe2x80x9d with NOx.
Accordingly, it is possible for the condition of a lean NOx trap deteriorate such that continued lean-burn operation either reduces overall vehicle fuel economy or increased overall vehicle emissions. What is needed, then, is a method and apparatus for controlling a lean-burn engine that prohibits lean engine operation when lean-burn operation is likely to have such a negative performance impact.
It is an object of the invention to provide a method and apparatus for controlling a lean-burn internal combustion engine of a motor vehicle to prohibit lean engine operation when such lean engine operation is likely to generate a negative performance impact, such as a reduced overall vehicle fuel economy or increase overall vehicle emissions.
In accordance with the invention, a method and apparatus is provided for controlling a lean-burn engine which prohibits lean-burn operation when a measure representing a performance impact, such as a determined measure of fuel economy benefit relative to stoichiometric engine operation, and a measure of trap NOx-storage efficiency, sampled once per trap fill/purge cycle at end of fill cycle, fall below respective calibratable threshold values. Preferably, the determination of the performance impact includes determining a relative cost due to periodically purging the trap of stored NOx, as well as the determination of the performance improvement likely to be obtained upon initiating a trap decontamination event, such as desulfurization of the trap.
Other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.